'slidechange' -> 'beforeslidechange' (closes #3003)
'slidechange' -> 'beforeslidechange' (closes #3003)

kenkunz/js-crash-course,hakimel/reveal.js,hakimel/reveal.js,kenkunz/js-crash-course,10xEngineer/reveal.js,lvh/reveal.js,lvh/reveal.js,10xEngineer/reveal.js